The present invention relates to an image forming apparatus and particularly to an electrophotographic apparatus of a type using a photosensitive screen having a number of small apertures.
In such an electrophotographic apparatus on the photosensitive screen there is formed an electrostatic latent image corresponding to a document to be copied. For this purpose the photosensitive screen is usually formed by applying on a conductive metal mesh a photoconductive layer, an insulating layer, a conductive layer, etc.
It has been known from, for example Japanese Patent Publication Nos. 30,320/70 and 11,579/74 and Japanese Patent Laid-Open Publication Nos. 84,640/73 and 59,840/73 to form an electrostatic latent image on a dielectric record paper by passing an ion stream through the fine apertures of the screen, said ion stream being modulated with various electric fields produced in or near the apertures or to form a colored image on a plain paper by selectively charging floating ink particles by means of the modulated ion stream.
Further in usual electrophotographic apparatus it has been widely practised that the photosensitive body is formed as a drum and a toner image on the photosensitive drum is transferred onto a plain paper or a latent image on the drum is transferred onto an electrostatic copy paper or a coated paper.
In thse known copying systems the photosensitive drum and a copying paper are traveled in an intimately contact manner and circumferential velocities of the drum and copy paper should be made equal to each other.
Further in the known systems an area or range where the transferring is effected is limited to an area in which the drum is substantially in contact with the transferring paper. This area is sometimes referred to as a printing or charging area.
It is also known from, for example Japanese Patent Publication No. 21,142/72 that the above mentioned photosensitive screen is formed as a drum and an image is formed on the record medium which travels through the printing area along a flat passage.
The present inventors have found that in order to effect an excellent copying with such an image forming apparatus of screen drum type a charging or printing width (measured in the traveling direction of record medium) produced by a printing corona charger must be very narrow. That is to say the width of the corona ion stream must be extremely small. If the printing width is made wide, image forming dots on the record medium are prolonged or expanded in the traveling direction of the record medium. Thus the definition or resolution of the copied image might be decreased. On the other hand if the printing width is made narrow, the printing speed has to be lowered and thus it is quite difficult to realize a high speed electrophotographic apparatus which is earnestly desired by many customers.
As explained above it is very difficult to find a solution which satisfies the above mentioned two mutually contradictory problems.